1. Field of the Invention
The present invention generally relates to a sheet conveyance unit used in an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions, and an image forming apparatus including the sheet conveyance unit.
2. Discussion of the Background Art
There are image forming apparatuses that form images on a single continuous long sheet of paper unreeled from a paper roll, transported by a sheet conveyance unit that typically includes a paper roll holder to support the paper roll and keep the sheet unreeled from the paper roll taut to transport the sheet reliably.
For example, JP-H11-246092-A and JP-S59-190151-A propose sheet conveyance units that include a tension guide provided with multiple elastic protrusions arranged like the teeth of a comb. The tension guide extends in a sheet width direction, perpendicular to a direction in which the long sheet is transported. Among the multiple elastic protrusions, those positioned in an area corresponding to the width of the sheet are pressed against the sheet, thus exerting a force to keep the sheet taut over the width of the sheet.
This approach, however, has several drawbacks. For example, because the protrusions are elastic, the force to keep the sheet taut exerted by each protrusion tends to fluctuate, skewing the sheet. Although JP-S59-190151-A states that the tension guide holds both ends of the sheet in the sheet width direction, thereby restricting the skew of the sheet, it is difficult to arrange the multiple elastic protrusions to accommodate all sheet sizes when the sheet conveyance unit accommodates domestic sheet sizes as well as foreign sheet sizes. More specifically, to accommodate both a domestic sheet size and a similar foreign sheet size, the elastic protrusions inevitably become very thin. Moreover, it is possible that the sheet is damaged or marked by the tooth-like protrusions pressed against the sheet.